Evolution of the aeronautical fixed telecommunication ...



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International Civil Aviation Organization

WORKING PAPER |ACP-WGW02/WP-7

2008-04-16

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AERONAUTICAL COMMUNICATIONS PANEL (ACP)

SECOND MEETING OF THE WORKING GROUP OF THE WHOLE

Montreal, Canada 21 – 25 April 2008

|Agenda Item 3: |Progress of work within WG-I (IPS), including potential future work program on IPS based air-ground and |

| |ground-ground applications. |

Evolution of the aeronautical fixed telecommunication network (AFTN)

By: CNS Section, ICAO Secretariat

(Presented by the ACP Secretary)

|SUMMARY |

|In this paper, a brief look is taken at the history of the aeronautical fixed service (AFS) ground-ground |

|messaging systems, and their related standards. An assessment is made of the current situation and a |

|potential way forward. Two conclusions are drawn based on the discussion in the paper; 1) that there may |

|be scope for developing new AFS G-G messaging system standards, for instance in support of the widely used|

|SMTP and XML protocols. 2) any new standardization work for G-G messaging systems should adopt an approach|

|of describing the minimum performance requirements, e.g. Addressing schemes and QOS parameters, rather |

|than describing the in depth technical details. Two recommendations are offered for consideration by the |

|panel. |

|ACTION |

|The ACP is invited to consider the recommendations contained in this paper. |

1. INTRODUCTION

1.1 This paper provides an assessment of the situation regarding development and evolution of aeronautical ground-ground messaging systems. In this paper, a brief look is taken at today’s implementation of AFTN, CIDIN, and AMHS systems; a brief assessment is made of SMTP which is the e-mail system specification used on the public Internet.; a conclusion is drawn that the standards regime for ground-ground messaging systems needs to be refocused towards required communication performance (RCP) characteristics; and finally, the ACP is invited to consider a few recommendations on a potential way forward.

2. BACKGROUND

2.1 The term “aeronautical fixed service (AFS)”has traditionally been used to refer to the infrastructure, systems or applications that are used for ground-ground (i.e. point-to-point or point-to-multipoint) voice and text/data communications. One of the major elements of the AFS is the Aeronautical Fixed Telecommunication Network (AFTN), an international network spanning the whole globe. This network is used for the transfer of aeronautical safety related messages such as flight plans and clearances. In the ensuing paragraphs, a look is taken at the evolution of this system, and the international standards and recommended practices (SARPs) which describe it.

2.1.1 The aeronautical fixed telecommunication network (AFTN) has been and still remains the main workhorse of the aeronautical fixed service (AFS). Despite its perceived shortcomings (e.g. low speed and obsolete technology), AFTN continues to provide global connectivity between all aeronautical entities and is, in fact, part of the aviation culture. The problems with the AFTN are:

a) dedicated low-speed circuits that make up the bulk of the network are increasingly not available/cost-effective any more;

b) emerging operational requirements (e.g. transmission of binary data, graphics and lower-case characters) cannot be supported;

c) it is based on a niche market that doesn’t lend itself to any modification or reconfiguration; and

d) due to the antiquated technology, it cannot be integrated with other and more modern communication systems or application.

2.1.2 Common ICAO data interchange network (CIDIN) that was introduced in Annex 10 in mid 1980s, was thought at that time to be the future backbone of aeronautical communications. However, it was only implemented in a limited scale (mainly in Europe and to a less extent in its neighbouring regions). Moreover, the implementations diverged in their specifications and plenty of in-house engineering has been needed to maintain CIDIN/CIDIN and CIDIN/AFTN communications. The “EUR CIDIN Manual”, published by the ICAO European Office, contains an up to date specification for CIDIN, and in order to avoid conflict with outdated CIDIN SARPs, all the technical details have been removed from Annex 10. Again, CIDIN is considered obsolete nowadays.

2.1.3 ATS message handling system (AMHS) was the next “future system” that was introduced by ICAO in the early 2000s. AMHS can be considered as the ground-ground element of the aeronautical telecommunication network (ATN) that has been tailored specifically for the purpose of message exchange. Several States/organizations have embarked on the implementation of the AMHS, though some of those implementations have deviated from the current ICAO provisions. AMHS is based on the ISO/OSI X.400 standard, a technology which has largely been abandoned by industry, in favour of the Simple Mail Transfer Protocol (SMTP), the protocol used for e-mail transfer on the public internet. Current implementation of X.400 based systems are mostly limited to military and AMHS. The current basic AMHS, unlike SMTP, does not support the transmission of binary data, graphics and so on. AMHS, like the AFTN, is a niche market, the number of potential system sales by developers probably being less than 200. Correspondingly, the pool of technical expertise available is also very small..

2.1.4 Due to the outdated technology or the limited capabilities of the currently standardized protocols for ground-ground messaging described above, many locally devised, non-Standardized schemes are also in use to provide text/data communications. Examples of these include AMHS over the Internet Protocol (IP) version 4 replacing the lower layers of the OSI stack through an IP Subnetwork Dependent Convergence Function (SNDCF), this approach deviates from ongoing standardization allowing the use of AMHS over the IP. Further examples include AFTN over TCP/IP, AFTN/SMTP gateways, and Internet-based web-mail applications used to send/receive AFTN messages. Many of these examples are supported today by the commercial AFTN/AMHS systems vendors.

2.1.5 Amendment proposals are in progress for Annexes 3 and 15 to permit the use of the Internet for exchange of MET and AIS information. In fact, many States are already using Internet for that purpose as it is the only medium that is widely available, fully capable of supporting all types of binary data (e.g. colours or graphics) and cost-effective. It is foreseen that with the increasing use of the Internet, the requirements for dedicated ground-ground communication systems will decrease.

2.2 Activities are now ongoing in ICAO to allow the use of the Internet Protocol Suite (IPS) for networking, as an alternative to the ATN/OSI. Unlike with the development of the ATN/OSI and the AMHS in the past, no new aviation specific standards were developed on the IPS as a means to transport air-ground and ground-ground data and voice. The new standards for the ATN using the IPS (ATN/IPS) will become applicable in November 2008, and guidance material will be published in the form of a manual by the end of 2008. The new SARPs consist only of high level requirements, and the Manual consist mainly of references and guidance to the Internet Society’s RFC’s. This approach makes the standards (RFCs) developed by the Internet Society (ISOC) available to aviation by reference, while providing minimal prescriptive guidance to ensure interoperability. A common addressing scheme for aviation is also being developed. The ATN/IPS is foreseen as a critical enabler for future air/ground as well as ground/ground communications.

3. FUTURE OF GROUND-GROUND COMMUNICATIONS

3.1 The communication technology is advancing at an extremely rapid rate. New products and services are constantly being offered to the public with unprecedented capabilities, performance and capacity with at ever decreasing costs.

3.2 The aviation community on the other hand insists on having its own dedicated and closed communication systems which tend to have insufficient capacities and are in general, outdated, expensive and of inferior performance. The reason for insisting on being separate from the public communications networks has generally been the need to ensure safety and security. This type of reasoning, however, is becoming increasingly questionable.

3.3 The idea of supporting the IPS native Simple Mail Transfer Protocol (SMTP) as an alternative / or evolutionary route beyond AMHS is not a new one. Obviously, the introduction of any new technology in the AFS would have to be made in a manner compatible with, and while not incurring significant additional costs to current and planned AFTN and AMHS system installations. One could foresee the use of SMTP systems within a region or a sub-region, while gateway functionality would have to be provided on the border towards AFTN or AMHS enabled regions. Region wide IP Networks in support of aviation are already being implemented. Unlike AFTN or AMHS based systems, with SMTP it is possible to reap the benefit of security by encryption, which is a standard feature in IP technology. Enterprise quality SMTP based solutions, even including full source code, are available at no cost, it would not take much effort to build an SMTP message switch in support of the AFS messaging needs. On the other hand, a typical AMHS switch costs in the range of USD $1 million to well above $2 million.  It is interesting to note that many of the current AMHS message switch providers already claim to support SMTP. An attachment is included with this paper, containing a list of “Frequently Asked Questions”(FAQ) and answers on the use of SMTP.

 

3.4 Discussions have also surfaced on the possible use of Extensible Markup Language (XML) as a standardized manner of formatting AFS messages.

3.5 Noting the above, there are two main options for the manner ICAO could continue its work on the Standardization in support of ground-ground communications:

a) continue to “catch up” with the technology, a process which has generally proven to be futile because in addition to the relatively long Standardization process, it often takes many years for any new system to become widely implemented. In other words, by the time a new communication system is widely implemented, it is most probably obsolete.

b) adopt a new Standardization regime wherein only high level requirements such as message formats, address management and quality of service (QoS) requirements are specified. This approach would facilitate timely adoption of modern technologies for message transfer, an example could be the SMTP case discussed above. An approach like this would keep the focus on the parameters essential to timely and error free transfer of messages end-to-end, regardless of the dissimilar networks which may be used underway.

3.6 Based on the points discussed so far, it can be concluded that option a) would lead to an ever more convoluted set of SARPs for different solutions which become increasingly more irrelevant with time. Option b) on the other hand, would firmly establish the requirements for safety and security without restricting the implementation options. It is noteworthy that currently there are no formal performance requirements for the ground-ground communication systems which is another point of concern.

4. cONCLUSION

4.1 Currently there is scope for developing standards based on SMTP in support of the Aeronautical Fixed Service (AFS), as an evolutionary means beyond the AMHS. SMTP being a technology built on the public Internet, is cheap and widely available. There have also been suggestions to standardize the AFS messaging format based on the Extended Markup Language (XML). These two examples serve as case in point that work needs to be done. On the other hand, it can be argued that there is no real need for ICAO to standardize technologies/protocols for aeronautical ground-ground communications. Unlike air-ground communications wherein interoperability (often in a limited spectrum environment) must be ensured with the clear definition of the signal-in-space, communications between aeronautical users on the ground is just like any other ground communications and doesn’t require any special techniques (e.g. for saving spectrum). What needs to be defined by ICAO is a system for defining message formats (for data), user addresses (e.g. based on location indicators and 3-letter designators) and Quality of Service (QoS) mapped to operational requirements.

5. ACTION BY THE MEETING

5.1 The ACP is invited to consider the following recommendations:

a) that an activity be initiated to revamp the provisions related to aeronautical ground-ground communications with the objectives of: reducing relevant Annex 10 provisions to a set of message formats (for data), addresses (based on current location indicators and 3-letter designators) and required communication performance (RCP) values.

b) that States and Regions which are already investing in AMHS, be advised to consider a modular approach, facilitating the inclusion of gateways to more modern technologies in their implementations to ensure a seamless path towards support of future data formats. This approach would also enable States which do not need to upgrade their current AFS messaging systems in the near-term to select a more modern and cost-effective solution when upgrading, e.g. SMTP.

— END —

Attachment

Subject: SMTP FAQ

Date: April 16, 2008

________________________________________________________________________

1. What is SMTP?

SMTP stands for Simple Mail Transfer Protocol, it is a store and forward method of composing, sending and storing messages over electronic communications system. It is the most widely used implementation for email on the public Internet.

2. Why use SMTP for aeronautical data transfer?

It is a commercially available product for data transfer over IP, and it is a much more affordable implementation, than the current ICAO mandated AMHS over IP or OSI.

3. What is the current ICAO Standard for store and forward messaging?

The current ICAO standard for store and forward method is Aeronautical Message Handling System (AMHS), based on X.400, running over OSI, it is defined in ICAO DOC 9705 Edition 3 and DOC 9880.

4. How long has SMTP been around?

The initial RFC which defined SMTP was RFC 788 dated CY1981. The current RFC which is the standard used by commercial vendors is RFCs 2821 and 2822 dated CY2000.

5. What countries currently support AFTN or AMHS through SMTP?

Specific implementations have been mentioned, but no real data exist. A survey may need to be sent out to get an official response from member states.

6. List vendors who have commercial SMTP servers?

Atmail, AXIGEN, Bongo, Citadel, Courier, Eudora Internet Mail Server, Exim, fmaild, Hexamail server, IBM Lotus Domino, Ipswitch IMail Server, Kerio MailServer, MailEnable Mail Server, Mailtraq, Mercury Mail Transport System, MeTA1 (successor of the sendmail X project), Microsoft Exchange Server, MMDF, Novell GroupWise, Novell NetMail, Open-Xchange, PostCast Server, Postfix, PostPath Email and Collaboration Server, PowerMTA, qmail, Scalix, Sendmail, SparkEngine, Sun Java System, WinGate, XMail, XMS Email Application Server, Zimbra, ZMailer, M-Switch From Isode Limited

7. What is X.400?

X.400 is a suite of ITU-T Recommendations that define standards for Data Communication Networks for Message Handling Systems (MHS) — more commonly known as "E-mail".

8. What is the cost of SMTP?

Enterprise hardened software is available online at no cost. Installation cost, including a server it would be about $5,000 USD. SMTP Mail Servers from commercial vendors are available in the $15,000-20,000 USD range.

9. How does the cost of SMTP compare with AMHS Message Transfer Agent (MTA)?

AMHS MTA are about $2,000,000 USD, SMTP server is about $20,000 USD.

10. How many known implementations of aeronautical AMHS exist in the world?

Asia/Pacific Region: 2

Car/SAM Region: 2 (Argentina, Paraguay)

Europe: 2 (Spain, Germany)

North America: 1 (USA)

11. Does SMTP run over IP or OSI?

SMTP runs over IP, X.400 is more closely associated to OSI, with RFC 1006 X.400 will run over IP.

12. Is SMTP a native IP application?

Yes, all the ITEF standards (RFC), are associated with IP implementation.

13. Can SMTP transfer binary data?

SMTP, unlike the current implementations of AMHS can transfer large binary attachments such as may be needed in support of Meteorological Services.

14. How will address resolution be handled with SMTP?

A DNS Server will be used. A DNS look-up of the Mail Exchange (MX) Record, corresponding to the domain name, is resolved to the SMTP Server IP address.

15. Will a DNS be required for SMTP?

Yes, it will be used to determine the SMTP Server IP address of the corresponding domain name, in the Mail Exchange (MX) record. This DNS does not need to be associated with the DNS of the public Internet.

16. Will SMTP run over IPv4 or IPv6?

SMTP will run over IPv4 or IPv6, the only issue is MX look-up, you made need to support look up tables of 32 bit for IPv4 and 128bit for IPv6.

17. How will the IP network be managed?

The approach is to manage the IP network as the AFTN network is managed by ICAO Regions.

18. Will SMTP be on the public IP Internet or a closed IP internet?

For security reasons, a closed ATN IPS network would be used, just as with AMHS/IPS, or with the current AFTN network.

19. How will a private ATN IP network be implemented?

ICAO Regions will determine who will support the backbone for that region and what countries will be the regional interfaces.

20. What security features are available for SMTP?

IPsec is optional for IPv4 and for IPv6 IPSec is a standard feature of the protocol.

21. What is the current position of ICAO and Regional Offices on SMTP?

No position, presently exist.

22. How will SMTP interface with AFTN and AMHS?

SMTP only needs to support an AFTN interface through a simple address translation, this will make it compatible to both AFTN and AMHS, since both support the AFTN interface.

23. What commercial AMHS vendors have shown interface capabilities to SMTP?

At least two of the prime vendors of aeronautical AMHS systems, Comsoft and Avitech, have provided documentation/presentations on interfaces to SMTP via their systems.

24. What are all the components needed for a SMTP implementation?

SMTP Server, client and a Gateway (SMTP/AFTN).

25 How is a message retrieved once it is at the destination SMTP Server?

Message access protocols exist the most popular are Post Office Protocol version 3(POP3), and Internet Message Access Protocol (IMAP).

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